linux-kernel - Re: [PATCH] x86/reboot: Avoid EFI reboot when not running on EFI

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Message-ID: <caa320fa61d619a85d6237076f5ec8ed134d11b7.camel@intel.com>
Date:   Thu, 29 Aug 2019 14:34:48 +0000
From:   "Bradford, Robert" <robert.bradford@...el.com>
To:     "tglx@...utronix.de" <tglx@...utronix.de>
CC:     "jgross@...e.com" <jgross@...e.com>,
        "riel@...riel.com" <riel@...riel.com>,
        "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jian-hong@...lessm.com" <jian-hong@...lessm.com>,
        "ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH] x86/reboot: Avoid EFI reboot when not running on EFI

On Thu, 2019-08-29 at 14:18 +0200, Thomas Gleixner wrote:
> On Thu, 29 Aug 2019, Rob Bradford wrote:
> 
> CC+ Ard
> 
> > Replace the check using efi_runtime_disabled() which only checks if
> > EFI
> > runtime was disabled on the kernel command line with a call to
> > efi_enabled(EFI_RUNTIME_SERVICES) to check if EFI runtime services
> > are
> > available.
> > 
> > In the situation where the kernel was booted without an EFI
> > environment
> > then only efi_enabled(EFI_RUNTIME_SERVICES) correctly represents
> > that no
> > EFI is available. Setting "noefi" or "efi=noruntime" on the
> > commandline
> > continue to have the same effect as
> > efi_enabled(EFI_RUNTIME_SERVICES)
> > will return false.
> > 
> > 
> > Signed-off-by: Rob Bradford <robert.bradford@...el.com>
> > ---
> >  arch/x86/kernel/reboot.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
> > index 09d6bded3c1e..0b0a7fccdb00 100644
> > --- a/arch/x86/kernel/reboot.c
> > +++ b/arch/x86/kernel/reboot.c
> > @@ -500,7 +500,7 @@ static int __init reboot_init(void)
> >  	 */
> >  	rv = dmi_check_system(reboot_dmi_table);
> >  
> > -	if (!rv && efi_reboot_required() && !efi_runtime_disabled())
> > +	if (!rv && efi_reboot_required() &&
> > efi_enabled(EFI_RUNTIME_SERVICES))
> 
> Why is efi_reboot_required() set at all in that situation?
> 
Hi Thomas,

This platform uses HW Reduced ACPI (
https://github.com/intel/cloud-hypervisor) but also supports direct
kernel boot without EFI.

/*
 * For most modern platforms the preferred method of powering off is
via
 * ACPI. However, there are some that are known to require the use of
 * EFI runtime services and for which ACPI does not work at all.
 *
 * Using EFI is a last resort, to be used only if no other option
 * exists.
 */
bool efi_reboot_required(void)
{
	if (!acpi_gbl_reduced_hardware)
		return false;

	efi_reboot_quirk_mode = EFI_RESET_WARM;
	return true;
}

This is a hardware workaround that assumes that all ACPI HW reduced
platforms do not support ACPI reset:

commit 44be28e9dd9880dca3e2cbf7a844f2114e67f2cb
Author: Matt Fleming <matt.fleming@...el.com>
Date:   Fri Jun 13 12:39:55 2014 +0100

    x86/reboot: Add EFI reboot quirk for ACPI Hardware Reduced flag
    
    It appears that the BayTrail-T class of hardware requires EFI in
order
    to powerdown and reboot and no other reliable method exists.
    
    This quirk is generally applicable to all hardware that has the
ACPI
    Hardware Reduced bit set, since usually ACPI would be the preferred
    method.
    
    Cc: Len Brown <len.brown@...el.com>
    Cc: Mark Salter <msalter@...hat.com>
    Cc: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
    Signed-off-by: Matt Fleming <matt.fleming@...el.com>

I'm reluctant to change the behaviour of efi_reboot_required() as it
might break older hardware whereas checking if we have an EFI runtime
is safer.

Rob